Redundancy of liens (paths) is an important technology to relieve communication lines (traffic) from failures in transmission lines and transfer apparatuses (which may include relay apparatuses). Redundancy technologies called linear protection and ring protection have been proposed or developed. These technologies, which are designed against failures in part of transmission lines or some transfer apparatuses, take a redundancy form called 1+1, 1:1, or the like.
Conventional redundancy technologies described in Japanese Laid-open Patent Publication Nos. 2006-49953, 2001-16240, 2011-171980, 2008-199284, and 2008-4993 are known.
In the technology in Japanese Laid-open Patent Publication No. 2006-49953, if a failure is detected in both an active (work) path and a standby (protection) path, which are hardware lines, a restoration path based on a software line is searched for and is set according to the search result, the restoration line being used in case of a failure.
In the technology in Japanese Laid-open Patent Publication No. 2001-16240, each two adjacent optical node apparatuses are mutually connected through first to third optical transfer paths; three rings formed with the first to third optical transfer paths are used to relieve a failed line.
In the technology in Japanese Laid-open Patent Publication No. 2011-171980, assuming a failure pattern in which a link of a first or second path is disconnected, a path controller calculates a third path with reference topology information.
In the technology in Japanese Laid-open Patent Publication No. 2008-199284, to allow for a failure in any one of two paths, another path is provided that can switch to a path in which a failure has not occurred without any interruption; the path free from a failure and the other path form a redundant architecture again without any interruption.
In the technology in Japanese Laid-open Patent Publication No. 2008-4993, to make a match between the protection modes of opposing apparatuses, a code that requests a mode to be changed is newly defined in an automatic protection switching (APS) byte; the APS byte that includes the code is transmitted to the distant apparatus.
However, the technologies described in Japanese Laid-open Patent Publication Nos. 2006-49953, 2001-16240, 2011-171980, 2008-199284, and 2008-4993 are susceptible to improvement in that how another path is activated quickly under simple control if a failure occurs in both the active path and the standby path and how a switchover to the other path is made.